1347-1350: Black Death sweeps across Europe, introduced into the bloodstream of its victims by a bacillus carried by the Oriental rat flea. When it is over, the epidemic has claimed more than 30 million people, approximately one-third of the continent's population.

1918: A worldwide influenza epidemic leaves 20 million dead.

1939-1945: World War II results in the loss of more than 50 million lives.

2000: To date, the AIDS virus has claimed approximately 17 million victims.

1926-1946: Controversial theories of the origin of AIDS abound. Some scientists believe the virus spread from monkeys to humans some time during this period.

1959: A man dies in Africa's Congo in what researchers say was the first proven AIDS death.

July 31, 1981: Under the headline "Rare Cancer Seen in 41 Homosexuals," the New York Times reports that a "rare and often rapidly fatal form of cancer" has been diagnosed in gay men in San Francisco and...